An organic light emitting diode (OLED), which is one of flat plate displays, is a representative electronic element that makes use of a mask (shadow mask). In recent years, the organic light emitting diode is widely developed and used as a display for personal mobile communication such as a cellular phone or the like. The organic light emitting diode is extremely thin and has an advantage that the organic light emitting diode can be addressed in the form of a matrix and can be driven even at a low voltage of 15 V or less.
When manufacturing such an organic light emitting diode, a mask (shadow mask) is typically used in order to form an organic material deposition region and a second electrode. In particular, when manufacturing a full-color organic light emitting diode including RGB pixels, the regions of the respective RGB pixels are formed using a mask. Thus, it is a matter of course that the resolution of the mask is high. The alignment of a substrate and a mask serves as a very important factor that determines the image quality of an organic light emitting diode.
Conventionally, when manufacturing a mask for an organic light emitting diode, a photoresist (PR) is coated on a metal sheet. A pattern is formed by an exposure and development process using a photomask (or a pattern is directly formed by an exposure device without any mask). Thereafter, the pattern is transferred to the metal sheet by etching, thereby manufacturing a mask.
As a metal sheet for the manufacture of a mask, copper, nickel, stainless steel or the like may be used. However, invar, which is a nickel-iron alloy, is often used as the metal sheet. Invar has an extremely low thermal linear expansion coefficient (1 to 2×10−6/° C.) comparable to a thermal linear expansion coefficient (3.20×10−6/° C.) of glass used as a substrate. Thus, the dimension of a mask with respect to a substrate is less likely to be changed by the influence of heat in a high-temperature deposition process. Accordingly, invar has an advantage in that it is possible to avoid misalignment between a deposition position at the time of registering a mask with a substrate and a deposition position of a deposition material at the time of high-temperature deposition.
However, invar has a problem in that a pattern cannot be easily formed by processing or etching. FIG. 1(A) is a sectional view showing a state in which the straightness of an inner wall of a transmission hole 10 formed at the time of etching is not secured and the inner width of the transmission hole 10 is not uniformly formed in an etching process for forming a pattern (d1≠d2). If depositing an organic light emitting material 20 as shown in FIG. 1(A), the organic light emitting material 20 is deposited on a substrate 30 in an actual deposition region 22 having a width d2 larger than a width d1 of a designed deposition region 24. The height of the organic light emitting material 20 is lower in the edge portion than in the central portion of the actual deposition region 22 having a width d2. Thus, there is generated a side effect that at the time of emitting light, the edge portion looks dimmer than the central portion. In this way, the mask made of invar has a limitation in that it is difficult to obtain a pattern having a resolution higher than a specific resolution.